1. Field of the Invention
This invention relates generally to glass surface treatment processes and more particularly to a method of preparing an optical glass surface for use in photography.
2. Description of the Prior Art
In the photographic industry it is common practice to use a contact printer to reproduce an image formed on one film negative on a second unexposed raw stock film and to use an enlarger to form an enlarged image of an image formed on a film negative. The conventional contact printer generally includes a transparent glass plate which the exposed film negative with the image formed thereon is placed with the emulsion side away from the glass plate. An unexposed raw stock film negative is placed on top of the exposed film with the emulsion side in contact with the exposed film. A transparent pressure platen is placed on top of the unexposed film to ensure that the two films are held in contact with the glass plate. A light is shown through the glass plate, exposed film, unexposed raw stock film and pressure platen to duplicate the image on the unexposed raw stock film.
The conventional enlarger also includes a transparent glass plate, upon which the film negative with the image to be enlarged is placed with the emulsion side away from the glass plate. A transparent pressure platen is placed on top of the exposed film to keep the exposed film in contact with the glass plate. The unexposed raw stock film upon which an enlarged image is to be reproduced is spaced above and in alignment with the exposed film. A light source shines through a condenser lenses located between the light source and the glass plate to pass substantially parallel light through the glass plate, exposed film, and glass platen. The light then passes through a projection lens located between the pressure platen and the unexposed raw stock film to enlarge the image. This enlarged image is then reproduced on the unexposed raw stock film.
It has been found that interference fringes, commonly known as Newton Rings, are formed between the glass plate and the abutting exposed film negative. The interference fringes are formed because of the wavey characteristics of the polished surface of the glass plate and the surface of the exposed film negative which causes wedges of air to be formed between the glass plate and film negative. Thus, as the light travels through the air wedges between the glass plate and the exposed film the optical path length for individual light rays will be different. This change in optical path length causes destructive interference between light rays when they are out of phase and constructive interference between light rays when they are in phase. This combination of destructive and constructive interference forms alternating light and dark rings, known as Newton Rings, between the glass plate and the film. When the image on the exposed film is duplicated on the raw stock film in the contact printer or the enlarger the interference fringes are also duplicated on the raw stock film. These interference fringes are obviously unwanted especially when high resolution is desired.
In the past, numerous methods have been used to breakup the interference fringes into a multiplicity of minute interference fringes which would not be duplicated on the raw stock film. One method has been to place a matted exposed film negative between the exposed film negative and the glass plate. This produces an irregular surface that breaks up the large interference fringes and forms a plurality of minute interference fringes that will not be reproduced. Another method has been to coat the glass plate with anti-reflection coatings which change the index of refraction of the surface of the glass plate so that the surface of the glass plate approaches the index of refraction of air. This technique supresses the intensity of the interference fringes and may also break up the large interference fringes and produce a multiplicity of smaller interference fringes. A further method has been to apply certain resins on the surface of the glass plate which again lowers the index of refraction of the surface so that it is as close as possible to the index refraction of air.
These prior art techniques perform the job of breaking up the large interference fringes into a multiplicity of smaller interference fringes adequately. However, they do have a number of distinct disadvantages. One disadvantage is that these methods are not particularly durable in that during the process of moving the exposed film negative across each surface the material degrades quite rapidly and consequently the matted exposed film, anti-reflection coatings and resins must be replaced frequently to produce the desired results. These methods also require constant checking and testing of the various coatings and the matted exposed film to detect when the degrading of the coatings and matted exposed film have reached the point where they no longer produce the multiplicity of smaller interference fringes. This, of course, is quite time consuming and costly and the production costs thereby increase enormously to the point where it is not economically feasibility to utilize contact printing or to make enlargements.